Carbon nanotubes, or so called `buckytubes`, were first observed by Iijima in 1991.sup.1. A method for macroscopic synthesis of nanotubes giving gram quantity of the material has been subsequently described.sup.2. Typically, the nanotubes consist of multilayers (normally 2-50) of concentric carbon tubes which are capped at both ends. The tubes are built up of sheets of carbon atoms arranged in hexagons and pentagons, with the pentagons concentrated in areas of low radius of curvature such as the tube ends. The tubes contain a hollow core up to 50 nm across typically 100-200 nm in length.
There has been much speculation on the possible physical and chemical properties of filled nanotubes, especially novel magnetic, mechanical, electrical, optical and catalytic properties.sup.3. However, no suitable method for opening and filling the carbon nanotubes with a variety of materials is currently available. There have previously been attempts to insert materials into the tubes by using composite metal-carbon electrodes during arc vaparisation.sup.4-9. These high temperature methods favour the production of metal carbides, the techniques can only be applied to a limited number of materials. In addition, the metal carbides or metal particles are encapsulated by carbon shells, and are thereby isolated from their environment. It has also been reported that if the nanotube is heated in air with lead, the tip of the tube can be opened, followed by the filling of the tube with the molten lead. About it of the tubes were filled in this manner and the presence of internal caps frequently limit the amount of tube filling. Recently, there have been two independent reports of the selective oxidation of nanotubes by carbon dioxide.sup.11 or oxygen.sup.12 in which ends of the nanotubes could be removed at temperatures &gt;600.degree. C. leaving a small portion 2-10%) of opened tubes. Oxidation in this manner also removes the outer layers of hexagonal carbon which form the body of the tubes. In consequence the tubes become thinned. In extremity they can lead to the formation of single layered tubes.sup.11. The filling of open tubes prepared by these gas phase oxidations has proved difficult, probably due to the presence of amorphous carbon blocking the tubes.sup.12. Here we describe a simple wet and highly selective method for the opening of the ends of nanotubes in very high yields, and the filling of these opened tubes with metal compounds.